Conventionally, a vehicle, running with a gas such as compressed natural gas (CNG) as fuel, supplies gas fuel, stored in a gas tank under high pressure, to the combustion chamber of an engine by regulating it to a predetermined pressure with a regulator. For example, Japanese Patent Laid-Open Publication No. 62-139720 discloses a technique in which such regulator is divided into a plural number and shows how they are disposed. In addition, Japanese Utility Model Laid-Open Publication No. 61-113967 discloses a technique in which a pressure switch is provided in fuel piping to warn when the fuel gas pressure lowers to less than a predetermined value.
In a vehicle using a gas as fuel, if the pressure of the gas supplied to the engine changes, the air/fuel ratio becomes impossible to control, causing abnormal combustion due to, for example, flame-out, over-lean or over-rich ratios. It is desirable to avoid such a situation. In addition, when abnormal sensing means is provided in the fuel piping for detecting an abnormal state, it is necessary to provide proper warning.
Furthermore, in the above-mentioned vehicle running with a gas such as compressed natural gas, the gas fuel stored in a gas tank under high pressure is supplied to the combustion chamber of the engine through fuel piping, and it is typical to dispose a fuel shut-off valve in the fuel piping. For example, a fuel shut-off device disclosed in Japanese Patent Publication No. 6-58092 opens a fuel shut-off valve provided in the fuel piping except when the start switch is opened (turned off) and the engine stops.
However, in the conventional gas fueled vehicle, since the fuel shut-off valve is always opened during operation of the engine to supply the gas fuel, the gas pressure in the fuel piping cannot be exactly detected if there is a slight change of pressure within an error allowed.
Moreover, to detect an abnormal state in the fuel supply system of a gas fueled vehicle, it is typical to provide a pressure sensor in the fuel supply path, and to determine an abnormal state with pressure change detected by the pressure sensor. A technique is also known in which, as disclosed in Japanese Patent Laid-Open Publication No. 61-272455, a fuel velocity sensor is provided in the fuel supply path to determine an abnormal state in the fuel system by change in flow velocity detected by the flow velocity sensor.
However, when an abnormal state is to be detected by the pressure sensor, it cannot be determined to be abnormal unless the range of fuel pressure reduction relates to maximum fuel consumption of the engine because otherwise detection is impossible since pressure change is too small. In addition, when an abnormal state is detected by the pressure sensor, the detection cannot be performed with high accuracy unless the fuel temperature is taken into consideration. The technique disclosed in Japanese Patent Laid-Open Publication No. 61-272455 basically has a similar problem.
Furthermore, a fuel injection unit of a vehicle is arranged to inject fuel by causing a current to flow through a solenoid coil of an injector under electronic control, and by moving a valve with magnetic force to form a gap with an injection nozzle. Japanese Patent Publication 63-35827, for example, is intended to shorten the operation time by supplying a large current in the initial stage when the fuel injection valve is opened, and then to quickly control the operation and switching characteristics of the fuel injection valve by reducing the current to a value not lower than the holding current.
However, in a case of the gas fueled vehicle, when the temperature decreases, water contained in oil and methane gas in a compressor when charging the gas and that generated at combustion tend to freeze so that there arises a situation where the fuel injection valve becomes difficult to open and the engine is difficult to be activated, particularly at its starting.
In addition, in the vehicle running with the gas such as the compressed natural gas as the fuel, the gas stored in the gas tank under high pressure of about 200 kg/cm.sup.2 is decompressed by a regulator to a predetermined pressure, and supplied to the engine. To avoid difficulty in supply of the fuel when the fuel is quickly cooled by adiabatic expansion due to decompression, the applicant has proposed a regulator as Japanese Patent Application No. 5-351891. The regulator prevents the problem due to quenching by introducing the cooling water heated through circulation in the engine.
Furthermore, regulators disclosed in Japanese Patent Laid-Open Publication Nos. 62-142853 and 62-170759 are also known. The technique disclosed in the above-mentioned Japanese Patent Laid-Open Publication No. 62-142853 holds constant the temperature of the regulator by a positive temperature coefficient (PTC) heater, the electrical resistance of which increases as the temperature rises. The technique disclosed in the above-mentioned Japanese Patent Laid-Open Publication No. 62-170759 maintains a temperature by introducing the heat in the exhaust system from a heat pipe to the regulator.
However, the Japanese Patent Laid-Open Publication Nos. 5-351891 and 62-170759 merely circulate the engine cooling water through the regulator, or introduce the heat from the exhaust system so that, when the temperature of engine cooling water, flow rate, or exhaust temperature changes, the temperature of the regulator changes accordingly so that the accuracy of pressure regulation tends to drift. In addition, when the PTC heater is used as in the Japanese Patent Laid-Open Publication No. 62-142853, the temperature of the regulator can be maintained constant, but the setting temperature cannot be changed easily.
A first object of the present invention is therefore to prevent defective regulation of the air/fuel ratio due to reduction of fuel pressure of a gas engine vehicle.
A second object of the present invention is to enable sensing of even slight variation of fuel pressure in a fuel piping of a gas engine vehicle so that countermeasures can be taken early.
A third object of the present invention is to accurately sense even a slight indication of an abnormal state in the fuel system of a gas engine vehicle.
A fourth object of the present invention is to prevent the situation where fuel cannot be supplied when the fuel injection valve is frozen when the engine at a low temperature is started in a cold district.
A fifth object of the invention is to properly hold the temperature of a regulator for regulating the pressure of fuel gas for a water-cooled gas engine vehicle so as to optimize the performance.